Copyright © 2007 jsd
The most important things to know about charge are:
The first ideas is related to all the others. For starters, there is only one charge-conservation law. If there were two kinds of charge, we would need two conservation laws. Similarly, charge enters in only one way in the Maxwell equations and in the Lorentz force law.
Note the contrast:
|There is only one kind of charge. You need only one variable to keep track of the charge.||There are lots of different charge-carrying particles. You need lots of variables if you want to keep track of the particles. However, that’s not charge. That’s the answer to a different question.|
|Again: To keep track of charge itself, you need only one number.||To keep track of the charged particles, you can sometimes get away with using only two numbers, but only approximately. The reality is more complicated, as discussed in section 3.|
|The amount of charge might be positive or it might be negative, but it can’t be both at the same time. It’s only one number.||You can have some number of positively-charged particles and negatively-charged particles at the same time.|
|Charge is an abstraction, not to be confused with the details of any particular charge-carrying particles.||Knowing the amount of charge will not tell you which type(s) of charged particles are present, nor how many.|
|Charge is charge. There is only one kind of charge.||There are lots of things we could talk about (e.g. momentum) that are not charge, not particle-number, and not even closely related to either one.|
It is sometimes useful to distinguish the weak two-component model from the strong two-component model, as discussed in section 2.1.
The one-component model is recommended. The strong two-component model is dead on arrival. The weak two-component model is consistent with the evidence, but it loses to the one-component model on the basis of simplicity and convenience.
It must be emphasized that for all practical purposes, the one-component model is the right way to think about things.
However, to be scientific, we must consider all the plausible hypotheses. If we do that, we find there are actually multiple versions of the two-component model, namely the weak version and the strong version.
This model is not provably wrong ... but I don’t recommend it. It is more complicated than the one-component model, but makes essentially the same predictions.
This model is provably wrong.
The fact is, as far as anybody can tell, there is only one
kind of charge. We only need one variable to describe charge.
However, in a great many textbooks and encyclopedias, one can find
statements alleging that:
«There are two kinds of charge, namely positive charge and negative charge.» ☠
|This expresses what is called the one-component model of electric charge, which is the recommended model.||This expresses the two-component model. We will have nothing good to say about it.|
|In reality, the charge can be represented as a point along a one-dimensional number line, as shown in figure 1. More positive charge is represented by points farther to the right on the number line, while more negative charge is represented by points farther to the left.||If we really did have two kinds of things – like apples and oranges – we would need two variables to describe the situation. The two different «charges» of any given object would be represented by a point on a two-dimensional coordinate plane, as in figure 2.|
|Positive charge cancels negative charge and vice versa.||Apples do not cancel oranges nor vice versa.|
|Figure 1: One-Dimensional Number Line||Figure 2: Two-Dimensional Number Plane|
|(e.g. Charge)||(e.g. Apples and Oranges)|
|The charge of the electron is different from the charge of the positron, but this is a difference in amount, not a difference in kind. The electron-charge is represented by a point on the number line, while the positron-charge is represented by another point on the same number line.|
|The key point is that only need one variable to describe the charge of any given object. At any given time, this variable may be positive, negative, or zero, but it is still just one variable.||If there were two kinds of charge, we would need two variables to keep track of them.|
Note: Some people use the word fluid, so they speak of a one-fluid model (as opposed to a two-fluid model). That’s essentially just another name for what we are calling the one-component model (as opposed to the two-component model). We prefer the latter name because charge isn’t exactly a fluid. It is an abstract quantity. It does, however, exhibit continuity of flow in close analogy to the flow of an indestructible fluid. Conservation of charge implies continuity of current. For more on what we mean by conservation and continuity, see section 5 and reference 1.
If you think of charge as a fluid, please do not imagine that it behaves like an «incompressible» fluid. In fact there are many situations where the density changes dramatically as the charge flows along. A video of this, and additional discussion, can be found in reference 1.
Also note the following contrast:
|In the real world, some quantities can be positive, negative, or zero. Consider for example elevation relative to sea level. If you look up the elevation of Furnace Springs, California, you will find it is a negative number.||To be sure, there are some quantitites that can never be negative, such as the number of apples in a basket. However, that does not change the fact that there are plenty of other quantities that can be positive, negative, or zero.|
Some textbooks use the two-fluid language only in passing, using it as nothing more than a figure of speech, which isn’t so bad. However, others go out of their way to argue that the one-component model is wrong and the two-fluid model is right, which is appalling.
It must be emphasized that keeping track of charged particles is more complicated than keeping track of charge itself. It’s the answer to a different question.
Keeping track of the particles is not evidence in favor of the two-component model, for the following reasons:
Bottom line: The existence of electrons and protons does not provide evidence in favor of the two-fluid model. The strong two-fluid model is dead on arrival. The weak two-fluid model is not provably wrong, but it has no advantages over the one-fluid model, and is unnecessarily complicated.
Within the field of atomic physics, there is a sub-field where we deal with three kinds of elementary particles: protons, neutrons, and electrons. These things are different in kind. To keep track of them, we need three numbers.
Interestingly, the three numbers that are conventionally used are not the simple particle-counts, but rather the following:
To fully specify an atom or atomic ion, it suffices to specify the three numbers (A, Z, q). For example:
If I tell you only that A=7 and Z=3, you have no idea what the charge is. You know it is lithium, but you have no idea whether it is ionized or not.
Conversely, if I tell you only that q=1, know the charge. The charge is 1 unit. You have no idea how many electrons, protons, or neutrons there are, but you know the charge. It could be Li+ or Na+ or Cu+ or lots of other things, but whatever it is, it bears one unit of charge.
To summarize: In this narrow sub-field, there are three kinds of elementary particles, but only one kind of charge. It takes three numbers to specify the particles, but it takes only one number to specify the charge.
More generally, there are dozens upon dozens of different kinds of particles, but still only one kind of electric charge. In physics, there are dozens of kinds of charged elementary particles but only one kind of electric charge; see figure 3. In chemistry, there are dozens upon dozens of charged atomic ions, not to mention molecular ions, but still only one kind of electric charge; see figure 4.
|In 1733, du Fay argued that there were two kinds of electricity, namely “vitreous” electricity and “resinous” electricity.|
|In 1747, William Watson argued in favor of a one-fluid model. Benjamin Franklin independently came to the same conclusion at the same time. Indeed, Franklin introduced the terms “positive”, “negative”, and “charge” for precisely this reason, to indicate a surplus or a deficit of the one type of electricity. See reference 4.||Ever since 1747, the weak two-fluid model has remained viable. It isn’t provably wrong. It is only marginally worse than the one-fluid model. In particular, we can restate the weak two-fluid model in modern terms: We classify the positive charge-carrying particles as «vitreous electricity» and classify the negative charge-carrying particles as «resinous electricty». This leaves us with a model that is impractical but not provably wrong.|
|The usual «proofs» of the two-fluid model are completely wrong, but that is not a sufficient reason to reject the model.|
|In 1865, the Maxwell equations were published. These equations give an extraordinarily good description of classical electromagnetism. This is relevant because if you think the Maxwell equations are right, then you think that charge is conserved. It’s a strict mathematical corollary.||This doesn’t settle the argument, because if you think that “vitreous” and “resinous” electricity are separately conserved, then as a corollary, the difference of the two (i.e. the net charge) is also conserved, as in equation 1a.|
|In 1932, the positron was discovered, confirming a prediction made by Dirac in 1928. This invalidates the strong two-fluid model. The number of electrons is not conserved. Protons are not the only carriers of “vitreous” electricity.|
|An additional stake was driven through the heart of the two-fluid model in 1964, when the subnuclear color charge was figured out. It is now very clear what an interaction looks like when it has to be described using more than one variable. See section 6. Note that color charge is completely independent of the ordinary electric charge. There is still only one kind of electric charge.|
So, for more than 80 years, it has been known that there are more than two types of charged particles, and the particles themselves are not separately conserved. The only electrical charge-like thing that is conserved is electrical charge itself.
Consider the following contrast:
One indispensable property of charge is its role in the law of conservation of charge. This is the starting point and the linchpin for any understanding of what “charge” means. The law of conservation of charge states that the amount of charge in a given region cannot change except by flowing across the boundary of the region. This law can be stated with great precision and formality, and has been extensively checked and confirmed. It stands on its own as a fundamental law of nature ... and it can also be derived as a corollary of the Maxwell equations. For more on what we mean by conservation, see reference 1.
Suppose we start with a negatively-charged pion, which decays into a muon, which decays into an electron. There are also some neutrinos involved, but let’s not worry about them. They carry no charge.
The point here is that we recognize the charge as being the same charge, even though it is being carried by different particles.
Let’s see how this law applies to the following scenario: We have a closed, isolated, electrically-insulated container. Within the container there is a number Np of protons; as always, they carry one unit of positive charge apiece. There is also a number Ne of electrons; as always, they carry one unit of negative charge apiece. Finally there is a number Nn of free neutrons; they are electrically neutral. Note that neutrons that are not bound in a nucleus are unstable and undergo beta-decay with a half-life of just over 10 minutes. In this subsection, we temporarily pretend that other types of charge carriers (such as positrons) do not exist.
If the two-fluid model were correct, we would in general need two variables to keep track of the two kinds of charge. In the previous paragraph we used two variables (Np and Ne), but that was to keep track of the particles, not the charge itself.
Let’s make a change of variables. For this system, we define
The variable Q has a conventional name: it is called the charge. Equation 1 tells us how to calculate the charge for this system (whereas for a more complicated system a more complicated formula might be needed). The value of Q can be positive, negative, or zero. The law of conservation of charge states that Q is conserved.1
In contrast, the law of conservation of charge says nothing about R. In fact, R has nothing to do with charge, and is not even a conserved quantity. Every time a neutron decays, R increases by two, since the decay produces a new proton and a new electron. The charge Q is conserved during this process, as it must be for any process … but R is manifestly not conserved.
Let’s be clear: we only need one variable, Q, to tell us everything we need to know about charge. Any one-variable model can always be dressed up to look like it has two (or more) variables, but there’s no point in doing so. It would be worse than useless.
Using the two variables Q and R is mathematically equivalent to using the two variables Np and Ne. If you start with Q, that is all you will ever need to keep track of charge; you don’t need R and you don’t need to keep track of Np and Ne separately in order to keep track of the amount of charge.
Of course R is meaningful; for example, an electrically-neutral plasma (Q=0, R≫0) has much greater electrical conductivity than an electrically-neutral vacuum (Q=0, R=0). (A parallel statement can be made concerning solid state physics: A compensated semiconductor is different from an intrinsic semiconductor.) That’s all fine; it simply tells you that there’s more to physics than just charge. Still, the fact remains that the single variable Q represents the charge, and you don’t need to know anything but Q to know the charge.
Tangential remark: It turns out that the quantity B := Np + Nn is conserved in this scenario. This is not a consequence of conservation of charge; it is a consequence of another conservation law, namely conservation of baryon number. Combining conservation of B with conservation of Q, we can infer that the quantity R + 2Nn is conserved … but R by itself is not conserved.
The nature of electrical charge can be understood with even greater clarity when contrasted with the subatomic color charge: There is only one kind of electrical charge, but there are three kinds of color charge.
Terminology: The term “charge” by itself is synonymous with plain old electrical charge. If you want to refer to color charge, you have to say “color charge”.
The symmetry of color charge is described by the group SU(3), while the symmetry of electrical charge is described by the group U(1). This allows us to say with great formality and precision that there are three kinds of color charge but only one kind of electrical charge.
For more on color charge, see reference 5, reference 6, and reference 7.
If we want to be scientific (as discussed in reference 8), due diligence requires that we examine the arguments in favor of the two-fluid model. We did some of this in section 4, but let’s go over it again.
One argument starts from the correct observation that in ordinary terrestrial matter, positive charge is predominantly embodied in protons, while negative charge is predominantly embodied in electrons. This however fails to prove that there are two kinds of charge. It fails for at least two reasons:
Charge, like other fundamental physical quantities such as energy and momentum, is something of an abstraction. The fact that it is abstract doesn’t make it any less real. (See reference 9 for a discussion of abstract things and embodiments thereof.) We don’t say that momentum embodied in a piece of wood is in any fundamental way different from momentum embodied in a piece of plastic; the embodiment is different, but the momentum is fundamentally the same. By the same token, the charge embodied in electrons is fundamentally the same as the charge embodied in other particles. In particular, negatively-charged pions, muons, antiprotons, etc., do not “contain” electrons. They contain negative charge, but they do not contain electrons. Conversely, there are a lot of things we know about an electron, including mass, lepton number, charge, et cetera. So we see that the concept of charge and the concept of electron are very different. Charge is just one property of the electron, one property among many.
This supports the crucial point of today’s discussion: If there were two kinds of charge (apples and oranges), you would need two numbers to describe the state of charge ... but this is never observed. You never need more than one number to describe the charge of any given object or region. To describe an electron you need more than one number, but that’s the answer to a different question. In particular, an electron is different from an antiproton, even though they have the same charge.
It’s a mistake to overemphasize the embodiments and/or the mobility of the embodiments. In a sample containing a mixture of five acids, there will be six different kinds of current. That alone should suffice to invalidate all the arguments in favor of a two-fluid model; if you’re going to allow more than one, logic requires you to allow an unlimited number.
The smart way to proceed is to say that there is only one kind of charge, just as there is only one kind of momentum. The momentum is the same kind of momentum, no matter whether it is embodied in electrons, protons, wood, plastic, or whatever. By the same token, the charge is the same kind of charge, no matter whether it is embodied in electrons, protons, ions, subatomic particles, or whatever.
Charge is not matter, and matter is not charge. If you mean “charge”, you should say “charge”, while if you mean “charged particle”, you should say “charged particle”. There exist many kinds of charged particles, but only one kind of electrical charge.
The situation is shown in figure 4.
|Within a given column, the entities on different rows have possibly different solubility, different lifetime, different mass, et cetera.||Within a given column, the entities have the same charge.|
|To keep track of many things, we need many numbers.||To keep track of charge, we need only one number.|
It would be unwise to even mention the two-fluid model in an introductory course. There is nothing to be gained from it. In general, mentioning a misconception is more likely to reinforce it than to dispel it.
Introductory courses often explore the topic of electricity by means of simple macroscopic experiments. These may include rubbing a rubber rod on fur or peeling bits of cellophane tape off the desktop, then testing for the presence of charge using home-made electroscopes. Alas, this does not provide any evidence for or against the two-fluid model or the one-fluid model. Even the fallacious arguments mentioned in section 7 are irrelevant in such a situation, because they involve microscopic phenomena that cannot be perceived by casual macroscopic observation.
However, there is more to physics than simple macroscopic experiments; there is also theory, and there are more advanced experiments. There are multiple ways to discriminate between the various hypotheses.
In an introductory class, there is no need to emphasize the one-component model, nor to refute the two-fluid model, nor even to mention the two-fluid model (unless a student brings it up). The sensible approach is just to introduce the idea of “charge”, say that the amount of charge Q can be positive, zero, or negative, and proceed from there. This is the approach taken by some textbooks (e.g. reference 10). There is no need to make things more complicated than that. The simple and obvious answer is the fundamentally correct answer.
Copyright © 2007 jsd